mimic

A NetHackmonster that ambushes unsuspecting players inside shops. It assumes the form of other items that are around it, so it is nearly impossible to detect before you stumble over it and it tries to kill you. Comes in three varieties: normal, large, and giant. The large and giant versions exude a sticky glue that makes escape impossible once the creature is engaged.

Mimics can occasionally be identified prior to the ambush by looking at the shop's merchandise carefully. An item that seems out of place (for example, a potion inside an armor dealership) is probably a mimic.

And yes, the shopkeeper will charge you for the corpse, as well as any items you break during the struggle.

Hokey but enjoyable 1997 movie directed by Guillermo del Toro, starring Mira Sorvino.
Manhattan entomologists create a genetically modified insect to kill cockroaches which are spreading a deadly disease. Three years later they discover these new critters have evolved the ability to mimic and hunt their only enemy, man.

I watched it, by coincidence, around the same time as I was reading The Mole People and was struck by parallels between the book and the film.

The Mole People is about homeless people living in tunnels underneath New York City, while Mimic is about giant insects who pass for human in the subways. There are some extraordinarily powerful scenes in the film, like when the insects' colony is found, all these creatures that look like men in overcoats are seen hanging from the ceiling of a densely crowded cave.

The most memorable aspect for me is the other bit of mimicry in the film: a little boy becomes obsessed with the creatures, who pass into and out of the world above through a portal located in a building opposite the boy's house. He has learned how to mimic the sound the creatures make and has christened one of them Mr. Funny Shoes.

Calvin Rankin was the only son of Ronald Rankin, a
genetics researcher. The nature of the elder Rankin's
experiments has never been fully revealed, but they were
such that when Calvin was accidentally exposed to some
of the chemicals being used, he gained the ability to
duplicate the skills of others when within 10 feet of
them. Upon discovering his son's new ability, Calvin's
father took him to an isolated cave far from anyone else
to protect his son from the obvious hysteria that would
ensue when an 8 year old was discovered to be able
to touch type, balance his check book, and change the oil
in a Buick. Why the elder Rankin chose a cave instead of
a double wide on an isolated piece of land was also never
fully explained.

Ronald Rankin then set about creating a machine for the
stated purpose of allowing Calvin to permanently retain
the abilities that duplicated. As Rankin was a genetics
expert and not an electrician, his machine shorted out
the power grid for the entire county. An angry mob gathered
and went to the cave, which calls into question what other
social factors were at play in this community that a power
outage is considered reason enough to break out the pitchforks
and torches. One does not envy the little league coach in
this community. The short circuit caused the machine to
become unstable and explode, sealing the cave and killing
the elder Rankin. Calvin escaped and fled, in fear for
his life.

Later, Ranking began to encounter members of the original
group of X-Men and found that his ability allowed him
to duplicate their abilities. He was invited to become
one of the team and did for a time under the name Mimic, becoming the only
member of the X-Men to not actually be a mutant. His
stay as a member was short, because after absobing all the
abilities of all of the members, he betrayed them and
fled back to the cave, hoping to use his father's machine
to permanently give him the powers of the X-Men. But
Calvin discovered that the machine was actually designed
to permanently remove his powers.

Soon, Mimic's powers began to grow and he began to
be a danger to himself and others. He contacted
Hank McCoy, the Beast, and sought his help in controlling
his powers. The two were interrupted by the Incredible
Hulk and Mimic began to absorb the radiation from
the Hulk in an attempt to kill himself. After a brief
time, he seemingly died and was left there by the Beast
and the Hulk.

It was later revealed that Rankin's power had locked
on to the signature of the mutant hero Wolverine and
duplicated his healing ability. After being healed, Rankin
continued to mimic Wolverine eventually taking on his
appearance down to his retractable claws. The two along
with the Hulk in his grey persona met and battled and
eventually, Rankin was able to return to "normal."
In recent years, Rankin has surfaced on a number of
occasions. He was one of the agents of the villain
Onslaught for a time and later joined other villains
as the Brotherhood of Evil Mutants. Also, the same character, albiet from an alternate reality, is part of the
dimension hopping team known as the Exiles.